Separated email accounts

Hi,

I assumed "5 external pop3 accounts" meant I could make 5 separated ....@mailbox.org email address accounts that me and others could log into with pop3 or imap to read and write mails separately.

Or, if I want to do this, should I first register a custom domain, and then make 5 accounts in that?

Or does "Standard plan: 50 aliases with custom domain" mean it is 1 email account with multiple aliases that cannot be separated with multiple Thunderbird imap connections?

Or is this not possible within 1 standard personal account :-) ?

Thanks,

Rob